The dehydration of N-(2-hydroxyethyl)-2-pyrrolidone (HEP) provides N-vinyl-2-pyrrolidone (NVP), a monomer used for making crosslinked or uncrosslinked polyvinylpyrrolidones, vinyl pyrrolidone-vinyl ester copolymers, and other valuable polymers. The polymers are used in beverage clarification, hair care, pharmaceutical tablet binding, and other industrial applications.
HEP is commercially available and is often made by reacting gamma-butyrolactone (GBL) with monoethanolamine (2-aminoethanol). It is normally purified by distillation (see U.S. Pat. No. 3,875,184 and U.S. Pat. Appl. Publ. 2002/0139657). The initial reaction product of GBL and 2-aminoethanol is N-(2-hydroxyethyl)-4-hydroxybutryamide, hereinafter “HEHBA,” which undergoes an intramolecular dehydration reaction to give HEP. Because HEHBA is not easily separated from HEP by distillation, even a distilled HEP product will normally contain traces of HEHBA. The distilled HEP product also usually contains traces of 2-pyrollidone (“2-Py”), a hydrolysis by-product.
HEHBA, 2-Py, and other HEP impurities are preferably minimized because they will eventually contaminate the desired N-vinyl-2-pyrrolidone monomer. Particularly for pharmaceutical applications, it is crucial to have very pure N-vinyl-2-pyrrolidone. Thus, the industry would benefit from better ways to purify HEP.
HEP, a liquid at room temperature, has a nominal freezing point of 20° C. We found that HEP will crystallize upon prolonged standing, and its purity level can be raised simply by separating the resulting solids from any remaining mother liquor. This observation prompted us to devise conditions under which HEP can be effectively and efficiently purified by crystallization.